Figured out why LSD is necessary

To train the aerobic system... since we're always in some ratio of aerobic/anerobic all the time, and we wanna train the aerobic system to flush lactic acid, we should build the aerobic capacity most. Or in muscle speak, build the slow twitch. Going too hard will involve the fast twitch too much.

Close. Your body is a chemical factory, nothing more. You can change what chemicals are made from what other chemicals through training. Your chemical factory will learn to do whatever it is that you make it do. I find that concept quite marvelous.

On an average group ride, you'll have a pyramid of work zones. A little fairly easy, a good bit of moderate, some hard, and a little very hard. If you ride nothing but moderate, you'll get dropped. If you ride all very hard, you won't last 10 miles.

So that's how you train, too. Building base builds aerobic systems. Then, when you go hard, even "anaerobic," a greater percentage of your power will come from aerobic chemical reactions, and less will be needed from anaerobic chemical reactions. This is, admittedly, a gross oversimplification but it does get at the core principle of training: specificity. You get better at what you train to do.

Fast twitch fibers contribute mostly to sprinting ability. Intervals or "going hard" efforts lasting over about seven minutes are training slow twitch fibers, or emphasizing the type IIa fast twitch fibers. That is because after about 5 minutes the fast twitch have packed it in and your slow twitch are all that's working.

We do burn lactic acid, or more correctly lactate, for aerobic energy. So that is one of those chemical processes that we improve when we train aerobically.

LSD is often translated as Long Slow Distance. I think that's a misnomer. You don't want to go slow, like zone 1 when you train, because that's not how you'll ride. Long fast distance is more like most riders' goal, so Long Steady Distance is a better translation.

All that said, if your riding goal is to do Alpe de Huez and nothing else, you still wouldn't do all your training at your climbing heart rate because even the principle of specificity is an oversimplification. It's complicated, which is why people hang out here, trying to learn more.

Carbonfiberboy is correct. To make another overgeneralized but still accurate for its purpose statement, it's actually not very easy to recruit fast twitch fibers. Similar to what cfb said, anything over 2 min starts to transition to slow twitch fibers, and eventually becomes almost entirely slow twitch by 5 min. For those first 2min of sprinting, your fast twitch fibers overpower the slow twitch.

Even your LT power sprints are mostly slow twitch. You'd have to get in the 200+% of your FTP to utilize fast twitch fibers.